1. Field of the Invention
The present invention relates to an image processing method and apparatus, and a recording medium. More particularly, the present invention relates to an image processing method for enhancing sharpness of an image, an image processing apparatus to which the image processing method is applied, and a recording medium on which a program for allowing a computer to function as the image processing apparatus is recorded.
2. Description of the Related Art
Enhancing sharpness of an image is very effective for enhancing image quality of digital image data. However, there is a problem that if a high frequency component or an intermediate/high frequency component of spatial frequency is simply enhanced in such a manner as in unsharp masking (USM) processing, noise components superimposed on the image data are also enhanced. Particularly, film image data obtained by reading a film image recorded on a photographic film includes intermediate-to-high frequency noise components resulting from graininess of the film (hereinafter referred to as “graininess components”). The graininess components are preferably not enhanced.
Accordingly, Japanese Patent Application (JP-A) No. 9-22460 discloses a technique in which, when sharpness is enhanced by separating image data into a low frequency component, an intermediate frequency component and a high frequency component, multiplying the intermediate and high frequency components by a gain which is individually set for each component, and synthesizing these frequency components multiplied by the gains, an evaluation value, such as a correlation between colors, is obtained for each pixel, as an evaluation value for evaluating the level of an image component to be enhanced (for example, an edge component in an image). The evaluation value is compared with a threshold value which is fixedly set, and a gain is set for each pixel, or a gain corresponding to the evaluation value is set for each pixel by means of a predetermined gain curve, which is determined by the relation between the evaluation value and the gain.
In JP-A No. 9-22460, various evaluation values are disclosed as evaluation values used to evaluate the level of an image component to be enhanced separately from a noise component such as the graininess component. However, there is a problem that, when any of evaluation values is used, the level of the noise components increases compared with the level of the image component to be enhanced. Accordingly, the accuracy of the evaluation value itself decreases, and it becomes difficult to properly evaluate the level of the image component to be enhanced.
The levels of the noise components superimposed on image data is not uniform for different portions of a single image. Therefore, if sharpness is enhanced by determining the gain based on a fixed predetermined threshold value or gain curve, high gains are set for portions in which the level of a noise component is high accordingly, and intermediate and high frequency components are enhanced. As a result, noise components are also partially enhanced, and poor image quality may be produced.